The present invention relates generally to lasers, detectors, and filters based on chiral structures, and more particularly to lasers, filters, and detectors utilizing multiple cholesteric liquid crystal elements having defects resulting in photonic band gap localized states, the defects being caused by twisting of one chiral structure relative to the other.
Semiconductor lasers have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players, in medical imaging, and in video displays. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter xe2x80x9cVCSELsxe2x80x9d) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs combine the performance advantages of LEDs and of edge-emitting lasers at costs comparable to LED solutions. VCSELs emit light vertically from the wafer surface, like LEDs, which means their fabrication and testing is fully compatible with standard I.C.s procedures and equipment, and also means that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) 2-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSEL has a ly symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELs suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive microfabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing large area arrays. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of low index contrast multilayer Bragg reflector and the gain cannot be improved in a VCSEL system as compared to that in an ordinary laser cavity. Also, an external device must be used to control the polarization of the light.
Other devices in common use in high tech industries include EM filters for blocking selected light frequencies or for blocking all frequencies except a very small group of frequencies in a narrow range. Such filters are particularly useful in telecommunication applications, such as in digital switches. Other commonly used EM devices include narrow EM detectors for detecting light at selected wavelengths.
In recent years, chiral materials, such as cholesteric liquid crystals have been used in a variety of lasing, filtering and other similar applications to address common drawbacks of standard semiconductor devices such as VCSELs. For example, a copending commonly assigned U.S. Patent Application xe2x80x9cChiral Laser Apparatus and Methodxe2x80x9d of Kopp et al. (Ser. No. 09/468,148) discloses a layered chiral structure laser with a defect formed by a light-emitting material layer. While that approach is advantageous to previously known techniques, it maybe difficult to construct a layered structure with a precise light emitting material thickness required to produce a defect (the required thickness must be equal to the wavelength of light in the medium divided by 4). More importantly, the position of the localized state caused by the defect cannot be easily controlled because the thickness of the light-emitting material cannot be changed once the device is manufactured. Similarly, previously known chiral filters only function at predefined frequencies.
It would thus be desirable to provide an apparatus and method for inducing a variable defect into a chiral structure. It would also be desirable to provide a tunable chiral EM filter and method. It would also be desirable to provide a chiral EM detector and method with a tunable detection band. It would further be desirable to provide a tunable chiral laser apparatus and method that has advantageous properties similar but superior to VCSELs and that has none of the VCSELs"" disadvantages.
This invention relates to use of chiral structures having a defect defined therein. In accordance with the present invention, a defect causing a localized state may be induced in a chiral structure composed of multiple chiral elements by twisting one element of the chiral structure with respect to the other elements along a common longitudinal axis such that directors of the element molecular layers that are in contact with one another are disposed at a particular xe2x80x9ctwistxe2x80x9d angle therebetween, the twist angle being greater than the shift angle between directors of consecutive layers.
The chiral twist structure may be advantageously utilized in a variety of applications. In one embodiment of the present invention the chiral twist structure may be used as a EM filter to filter light emitted by an external light source at particular wavelengths and circular polarizations. With the addition of a tuning device, the twist angle of the chiral twist structure may be changed resulting in changing the operational wavelength and wavelengths ranges of the filterxe2x80x94essentially producing an readily tunable EM filter.
In another embodiment of the present invention, the chiral twist structure may be used as an EM detector to detect certain wavelengths of light emitted by an external light source. The EM detector is configured similarly to the EM filter except that a sensing element is positioned at the location of the twist and connected to an external detector device. With the addition of a tuning device, the twist angle of the chiral twist structure may be changed resulting in a change of the operational wavelength the filter and the detectorxe2x80x94essentially producing an readily tunable EM filter or detector.
In yet another embodiment of the invention, the chiral structure is configured as a laser and includes a gain producing light-emitting material layer connected to an excitation source which excites the light-emitting layer to produce lasing at a predefined wavelength. With the addition of a tuning device, the twist angle of the chiral twist structure may be changed resulting in changing the operational wavelength and wavelengths ranges of the detectorxe2x80x94essentially producing a readily tunable chiral laser. In a variation of the chiral laser embodiment, instead of a separate light-emitting material layer, the chiral elements may be doped with excitable light-emitting materials.
In accordance with the present invention, more than two chiral elements may be utilized to configure a filter, detector or laser capable of simultaneous operation at more than one tunable wavelength or wavelength range. The chiral twist structure may also be arranged in a post configuration having transverse dimensions smaller than its height.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.